This invention relates to an electric motor driven brake pressure modulator for a motor vehicle braking system.
The use of motor driven pressure modulators in a vehicle braking system is known. For example, such use includes a motor driven braking pressure modulator in vehicle antilock braking systems. In these systems, the brake pressure is modulated by means of a DC torque motor driving a piston in a cylinder whose volume is modulated to control the hydraulic pressure at the wheel brake. In one such known system, the motor is controlled to position the piston in an initial, fully extended home position at which a check valve is unseated to couple the brake system master cylinder to the wheel brake to allow normal braking. When antilock brake pressure modulation is required, the motor retracts the piston (which allows a check valve to close to isolate the master cylinder from the wheel brake) to reduce brake pressure and thereafter modulates the piston position to provide pressure control for antilock braking. When antilock braking is no longer required, the motor returns the piston to its extended home position. Typically in these prior systems, the braking pressure is established based upon a relationship between a motor current, motor torque and the hydraulic pressure acting on the head of the piston. Motor current then becomes the controlled parameter to establish a desired braking condition via the brake pressure/motor current relationship.
The control of braking pressure in such systems wherein a brake pressure actuator is controlled by a motor may be substantially simplified by controlling the pressure based upon the position of the actuator controlled by the electric motor as opposed to motor current. This is because the functional relationship between position and pressure is substantially simpler than the functional relationship between motor current and pressure due to the motor dynamics. However, this would necessitate the addition of a position sensor on the actuator in order to establish the positional information.